Copyright Notice

Abstract

This memo offers non-normative implementation guidance for the Real-
time Protocol (RTP) MIDI (Musical Instrument Digital Interface)
payload format. The memo presents its advice in the context of a
network musical performance application. In this application two
musicians, located in different physical locations, interact over a
network to perform as they would if located in the same room.
Underlying the performances are RTP MIDI sessions over unicast UDP.
Algorithms for sending and receiving recovery journals (the
resiliency structure for the payload format) are described in detail.
Although the memo focuses on network musical performance, the
presented implementation advice is relevant to other RTP MIDI
applications.

However, [RFC4695] does not define algorithms for sending and
receiving MIDI streams. Implementors are free to use any sending or
receiving algorithm that conforms to the normative text in [RFC4695],
[RFC3550], [RFC3551], and [MIDI].

In this memo, we offer implementation guidance on sending and
receiving MIDI RTP streams. Unlike [RFC4695], this memo is not
normative.

RTP is a mature protocol, and excellent RTP reference materials are
available [RTPBOOK]. This memo aims to complement the existing
literature by focusing on issues that are specific to the MIDI
payload format.

The memo focuses on one application: two-party network musical
performance over wide-area networks, following the interoperability
guidelines in Appendix C.7.2 of [RFC4695]. Underlying the
performances are RTP MIDI sessions over unicast UDP transport.
Resiliency is provided by the recovery journal system [RFC4695]. The
application also uses the RTP Control Protocol (RTCP, [RFC3550]).

The application targets a network with a particular set of
characteristics: low nominal jitter, low packet loss, and occasional
outlier packets that arrive very late. However, in Section 6.2 of
this memo, we discuss adapting the application to other network
environments.

As defined in [NMP], a network musical performance occurs when
musicians located at different physical locations interact over a
network to perform as they would if located in the same room.

2. Starting the Session

In this section, we describe how the application starts a two-player
session. We assume that the two parties have agreed on a session
configuration, embodied by a pair of Session Description Protocol
(SDP, [RFC4566]) session descriptions.

One session description (Figure 1) defines how the first party wishes
to receive its stream. The other session description (Figure 2)
defines how the second party wishes to receive its stream.

The session description in Figure 1 codes that the first party
intends to receive a MIDI stream on IP4 number 192.0.2.94 (coded in
the c= line) at UDP port 16112 (coded in the m= line). Implicit in
the SDP m= line syntax [RFC4566] is that the first party also intends
to receive an RTCP stream on 192.0.2.94 at UDP port 16113 (16112 +
1). The receiver expects that the PT field of each RTP header in the
received stream will be set to 96 (coded in the m= line).

Likewise, the session description in Figure 2 codes that the second
party intends to receive a MIDI stream on IP4 number 192.0.2.105 at
UDP port 5004 and intends to receive an RTCP stream on 192.0.2.105 at

UDP port 5005 (5004 + 1). The second party expects that the PT RTP
header field of received stream will be set to 101.

Figure 2. Session description for second participant

The session descriptions use the mpeg4-generic media type (coded in
the a=rtpmap line) to specify the use of the MPEG 4 Structured Audio
renderer [MPEGSA]. The session descriptions also use parameters to
customize the stream (Appendix C of [RFC4695]). The parameter values
are identical for both parties, yielding identical rendering
environments for the two client hosts.

The bandwidth (b=) AS parameter [RFC4566] [RFC3550] indicates that
the total RTP session bandwidth is 20 kbs. This value assumes that
the two players send 10 kbs streams concurrently. To derive the 10
kbs value, we begin with the analysis of RTP MIDI payload bandwidth
in Appendix A.4 of [NMP] and add in RTP and IP4 packet overhead and a
small safety factor.

The bandwidth RR parameter [RFC3556] indicates that the shared RTCP
session bandwidth for the two parties is 400 bps. We set the
bandwidth SR parameter to 0 bps, to signal that sending parties and
non-sending parties equally share the 400 bps of RTCP bandwidth.
(Note that in this particular example, the guardtime parameter value
of 44100 ensures that both parties are sending for the duration of
the session.) The 400 bps RTCP bandwidth value supports one RTCP
packet per 5 seconds from each party, containing a Sender Report and
CNAME information [RFC3550].

We now show an example of code that implements the actions the
parties take during the session. The code is written in C and uses
the standard network programming techniques described in [STEVENS].
We show code for the first party (the second party takes a symmetric
set of actions).

Figure 3 shows how the first party initializes a pair of socket
descriptors (rtp_fd and rtcp_fd) to send and receive UDP packets.
After the code in Figure 3 runs, the first party may check for new
RTP or RTCP packets by calling recv() on rtp_fd or rtcp_fd.

Applications may use recv() to receive UDP packets on a socket using
one of two general methods: "blocking" or "non-blocking".

A call to recv() on a blocking UDP socket puts the calling thread to
sleep until a new packet arrives.

A call to recv() on a non-blocking socket acts to poll the device:
the recv() call returns immediately, with a return value that
indicates the polling result. In this case, a positive return value
signals the size of a new received packet, and a negative return
value (coupled with an errno value of EAGAIN) indicates that no new
packet was available.

The choice of blocking or non-blocking sockets is a critical
application choice. Blocking sockets offer the lowest potential
latency (as the OS wakes the caller as soon as a packet has arrived).
However, audio applications that use blocking sockets must adopt a
multi-threaded program architecture, so that audio samples may be
generated on a "rendering thread" while the "network thread" sleeps,
awaiting the next packet. The architecture must also support a
thread communication mechanism, so that the network thread has a
mechanism to send MIDI commands the rendering thread.

In contrast, audio applications that use non-blocking sockets may be
coded using a single thread, that alternates between audio sample
generation and network polling. This architecture trades off
increased network latency (as a packet may arrive between polls) for
a simpler program architecture. For simplicity, our example uses
non-blocking sockets and presumes a single run loop. Figure 4 shows
how the example configures its sockets to be non-blocking.

Figure 5 shows how to use recv() to check a non-blocking socket for
new packets.

The first party also uses rtp_fd and rtcp_fd to send RTP and RTCP
packets to the second party. In Figure 6, we show how to initialize
socket structures that address the second party. In Figure 7, we
show how to use one of these structures in a sendto() call to send an
RTP packet to the second party.

Note that the code shown in Figures 3-7 assumes a clear network path
between the participants. The code may not work if firewalls or
Network Address Translation (NAT) devices are present in the network
path.

3. Session Management: Session Housekeeping

After the two-party interactive session is set up, the parties begin
to send and receive RTP packets. In Sections 4-7, we discuss RTP
MIDI sending and receiving algorithms. In this section, we describe
session "housekeeping" tasks that the participants also perform.

One housekeeping task is the maintenance of the 32-bit
Synchronization Source (SSRC) value that uniquely identifies each
party. Section 8 of [RFC3550] describes SSRC issues in detail, as
does Section 2.1 in [RFC4695]. Another housekeeping task is the
sending and receiving of RTCP. Section 6 of [RFC3550] describes RTCP
in detail.

Another housekeeping task concerns security. As detailed in the
Security Considerations section of [RFC4695], per-packet
authentication is strongly recommended for use with MIDI streams,
because the acceptance of rogue packets may lead to the execution of
arbitrary MIDI commands.

A final housekeeping task concerns the termination of the session.
In our two-party example, the session terminates upon the exit of one
of the participants. A clean termination may require active effort
by a receiver, as a MIDI stream stopped at an arbitrary point may
cause stuck notes and other indefinite artifacts in the MIDI
renderer.

The exit of a party may be signalled in several ways. Session
management tools may offer a reliable signal for termination (such as
the SIP BYE method [RFC3261]). The (unreliable) RTCP BYE packet
[RFC3550] may also signal the exit of a party. Receivers may also
sense the lack of RTCP activity and timeout a party or may use
transport methods to detect an exit.

4. Sending Streams: General Considerations

In this section, we discuss sender implementation issues.

The sender is a real-time data-driven entity. On an ongoing basis,
the sender checks to see if the local player has generated new MIDI
data. At any time, the sender may transmit a new RTP packet to the
remote player for the reasons described below:

New MIDI data has been generated by the local player, and the
sender decides that it is time to issue a packet coding the data.

The local player has not generated new MIDI data, but the sender
decides that too much time has elapsed since the last RTP packet
transmission. The sender transmits a packet in order to relay
updated header and recovery journal data.

In both cases, the sender generates a packet that consists of an RTP
header, a MIDI command section, and a recovery journal. In the first
case, the MIDI list of the MIDI command section codes the new MIDI
data. In the second case, the MIDI list is empty.

Figure 7. Using sendto() to send an RTP packet

Figure 8 shows the 5 steps a sender takes to issue a packet. This
algorithm corresponds to the code fragment for sending RTP packets
shown in Figure 7 of Section 2. Steps 1, 2, and 3 occur before the
sendto() call in the code fragment. Step 4 corresponds to the
sendto() call itself. Step 5 may occur once Step 3 completes.

The algorithm for Sending a Packet is as follows:

Generate the RTP header for the new packet. See Section 2.1 of
[RFC4695] for details.

Generate the MIDI command section for the new packet. See Section
3 of [RFC4695] for details.

Generate the recovery journal for the new packet. We discuss this
process in Section 5.2. The generation algorithm examines the
Recovery Journal Sending Structure (RJSS), a stateful coding of a
history of the stream.

Send the new packet to the receiver.

Update the RJSS to include the data coded in the MIDI command
section of the packet sent in step 4. We discuss the update
procedure in Section 5.3.

Figure 8. A 5 step algorithm for sending a packet

In the sections that follow, we discuss specific sender
implementation issues in detail.

4.1. Queuing and Coding Incoming MIDI Data

Simple senders transmit a new packet as soon as the local player
generates a complete MIDI command. The system described in [NMP]
uses this algorithm. This algorithm minimizes the sender queuing
latency, as the sender never delays the transmission of a new MIDI
command.

In a relative sense, this algorithm uses bandwidth inefficiently, as
it does not amortize the overhead of a packet over several commands.
This inefficiency may be acceptable for sparse MIDI streams (see
Appendix A.4 of [NMP]). More sophisticated sending algorithms
[GRAME] improve efficiency by coding small groups of commands into a
single packet, at the expense of increasing the sender queuing
latency.

Senders assign a timestamp value to each command issued by the local
player (Appendix C.3 of [RFC4695]). Senders may code the timestamp
value of the first MIDI list command in two ways. The most efficient
method is to set the RTP timestamp of the packet to the timestamp
value of the first command. In this method, the Z bit of the MIDI
command section header (Figure 2 of [RFC4695]) is set to 0, and the
RTP timestamps increment at a non-uniform rate.

However, in some applications, senders may wish to generate a stream
whose RTP timestamps increment at a uniform rate. To do so, senders
may use the Delta Time MIDI list field to code a timestamp for the
first command in the list. In this case, the Z bit of the MIDI
command section header is set to 1.

Senders should strive to maintain a constant relationship between the
RTP packet timestamp and the packet sending time: if two packets have
RTP timestamps that differ by 1 second, the second packet should be
sent 1 second after the first packet. To the receiver, variance in
this relationship is indistinguishable from network jitter. Latency
issues are discussed in detail in Section 6.

Senders may alter the running status coding of the first command in
the MIDI list, in order to comply with the coding rules defined in
Section 3.2 of [RFC4695]. The P header bit (Figure 2 of [RFC4695])
codes this alteration of the source command stream.

4.2. Sending Packets with Empty MIDI Lists

During a session, musicians might refrain from generating MIDI data
for extended periods of time (seconds or even minutes). If an RTP
stream followed the dynamics of a silent MIDI source and stopped
sending RTP packets, system behavior might be degraded in the
following ways:

The receiver's model of network performance may fall out of date.

Network middleboxes (such as Network Address Translators) may
"time-out" the silent stream and drop the port and IP association
state.

If the session does not use RTCP, receivers may misinterpret the
silent stream as a dropped network connection.

Session participants may specify the frequency of keep-alive packets
during session configuration with the MIME parameter "guardtime"
(Appendix C.4.2 of [RFC4695]). The session descriptions shown in
Figures 1-2 use guardtime to specify a keep-alive sending interval of
1 second.

Senders may also send empty packets to improve the performance of the
recovery journal system. As we describe in Section 6, the recovery
process begins when a receiver detects a break in the RTP sequence
number pattern of the stream. The receiver uses the recovery journal
of the break packet to guide corrective rendering actions, such as
ending stuck notes and updating out-of-date controller values.

Consider the situation where the local player produces a MIDI NoteOff
command (which the sender promptly transmits in a packet) but then 5
seconds pass before the player produces another MIDI command (which
the sender transmits in a second packet). If the packet coding the
NoteOff is lost, the receiver is not aware of the packet loss
incident for 5 seconds, and the rendered MIDI performance contains a
note that sounds for 5 seconds too long.

To handle this situation, senders may transmit empty packets to
"guard" the stream during silent sections. The guard packet
algorithm defined in Section 7.3 of [NMP], as applied to the
situation described above, sends a guard packet after 100 ms of
player inactivity, and sends a second guard packet 100 ms later.
Subsequent guard packets are sent with an exponential backoff, with a
limiting period of 1 second (set by the "guardtime" parameter in
Figures 1-2). The algorithm terminates once MIDI activity resumes,
or once RTCP receiver reports indicate that the receiver is up to
date.

The perceptual quality of guard packet-sending algorithms is a
quality of implementation issue for RTP MIDI applications.
Sophisticated implementations may tailor the guard packet sending
rate to the nature of the MIDI commands recently sent in the stream,
to minimize the perceptual impact of moderate packet loss.

As an example of this sort of specialization, the guard packet
algorithm described in [NMP] protects against the transient artifacts
that occur when NoteOn commands are lost. The algorithm sends a
guard packet 1 ms after every packet whose MIDI list contains a
NoteOn command. The Y bit in Chapter N note logs (Appendix A.6 of
[RFC4695]) supports this use of guard packets.

Congestion control and bandwidth management are key issues in guard
packet algorithms. We discuss these issues in the next section.

4.3. Congestion Control and Bandwidth Management

The congestion control section of [RFC4695] discusses the importance
of congestion control for RTP MIDI streams and references the
normative text in [RFC3550] and [RFC3551] that concerns congestion
control. To comply with the requirements described in those
normative documents, RTP MIDI senders may use several methods to
control the sending rate:

As described in Section 4.1, senders may pack several MIDI
commands into a single packet, thereby reducing stream bandwidth
(at the expense of increasing sender queuing latency).

Guard packet algorithms (Section 4.2) may be designed in a
parametric way, so that the tradeoff between artifact reduction
and stream bandwidth may be tuned dynamically.

The recovery journal size may be reduced by adapting the
techniques described in Section 5 of this memo. Note that in all
cases, the recovery journal sender must conform to the normative
text in Section 4 of [RFC4695].

The incoming MIDI stream may be modified to reduce the number of
MIDI commands without significantly altering the performance.
Lossy "MIDI filtering" algorithms are well developed in the MIDI
community and may be directly applied to RTP MIDI rate management.

RTP MIDI senders incorporate these rate control methods into feedback
systems to implement congestion control and bandwidth management.
Sections 10 and 6.4.4 of [RFC3550] and Section 2 in [RFC3551]
describe feedback systems for congestion control in RTP, and Section
6 of [RFC4566] describes bandwidth management in media sessions.

5. Sending Streams: The Recovery Journal

In this section, we describe how senders implement the recovery
journal system. The implementation we describe uses the default
"closed-loop" recovery journal semantics (Appendix C.2.2.2 of
[RFC4695]).

We begin by describing the Recovery Journal Sending Structure (RJSS).
Senders use the RJSS to generate the recovery journal section for RTP
MIDI packets.

The RJSS is a hierarchical representation of the checkpoint history
of the stream. The checkpoint history holds the MIDI commands that
are at risk to packet loss (Appendix A.1 of [RFC4695] precisely
defines the checkpoint history). The layout of the RJSS mirrors the
hierarchical structure of the recovery journal bitfields.

Figure 9 shows an RJSS implementation for a simple sender. The leaf
level of the RJSS hierarchy (the jsend_chapter structures)
corresponds to channel chapters (Appendices A.2-9 in [RFC4695]). The
second level of the hierarchy (jsend_channel) corresponds to the
channel journal header (Figure 9 in [RFC4695]). The top level of the
hierarchy (jsend_journal) corresponds to the recovery journal header
(Figure 8 in [RFC4695]).

Each RJSS data structure may code several items:

The current contents of the recovery journal bitfield associated
with the RJSS structure (jheader[], cheader[], or a chapter
bitfield).

A seqnum variable. Seqnum codes the extended RTP sequence number
of the most recent packet that added information to the RJSS
structure. If the seqnum of a structure is updated, the seqnums
of all structures above it in the recovery journal hierarchy are
also updated. Thus, a packet that caused an update to a specific
jsend_chapter structure would update the seqnum values of this
structure and of the jsend_channel and jsend_journal structures
that contain it.

Ancillary variables used by the sending algorithm.

A seqnum variable for a level is set to zero if the checkpoint
history contains no information at the level of the seqnum variable,
and no information at any level below the level of the seqnum
variable. This coding scheme assumes that the first sequence number
of a stream is normalized to 1, and limits the total number of stream
packets to 2^32 - 1.

The cm_unused and ch_never parameters in Figures 1-2 define the
subset of MIDI commands supported by the sender (see Appendix C.2.3
of [RFC4695] for details). The sender transmits most voice commands
but does not transmit system commands. The sender assumes that the
MIDI source uses note commands in the typical way. Thus, the sender
does not use the Chapter E note resiliency tools (Appendix A.7 of
[RFC4695]). The sender does not support Control Change commands for
controller numbers with All Notes Off (123-127), All Sound Off (120),
and Reset All Controllers (121) semantics and does not support
enhanced Chapter C encoding (Appendix A.3.3 of [RFC4695]).

We chose this subset of MIDI commands to simplify the example. In
particular, the command restrictions ensure that all commands are
active, that all note commands are N-active, and that all Control
Change commands are C-active (see Appendix A.1 of [RFC4695] for
definitions of active, N-active, and C-active).

In the sections that follow, we describe the tasks a sender performs
to manage the recovery journal system.

5.1. Initializing the RJSS

At the start of a stream, the sender initializes the RJSS. All
seqnum variables are set to zero, including all elements of
note_seqnum[] and control_seqnum[].

The sender initializes jheader[] to form a recovery journal header
that codes an empty journal. The S bit of the header is set to 1,
and the A, Y, R, and TOTCHAN header fields are set to zero. The
checkpoint packet sequence number field is set to the sequence number
of the upcoming first RTP packet (per Appendix A.1 of [RFC4695]).

In jsend_chaptern, elements of note_tstamp[] are set to zero. In
jsend_chaptern and jsend_chapterc, elements of bitfield_ptr[] are set
to the null pointer index value (bitfield_ptr[] is an array whose
elements point to the first octet of the note or control log
associated with the array index).

5.2. Traversing the RJSS

Whenever an RTP packet is created (Step 3 of the algorithm defined in
Figure 8), the sender traverses the RJSS to create the recovery
journal for the packet. The traversal begins at the top level of the
RJSS. The sender copies jheader[] into the packet and then sets the
S bit of jheader[] to 1.

The traversal continues depth-first, visiting every jsend_channel
whose seqnum variable is non-zero. The sender copies the cheader[]
array into the packet and then sets the S bit of cheader[] to 1.
After each cheader[] copy, the sender visits each leaf-level chapter,
in the order of its appearance in the chapter journal Table of
Contents (first P, then C, then W, then N, as shown in Figure 9 of
[RFC4695]).

If a chapter has a non-zero seqnum, the sender copies the chapter
bitfield array into the packet and then sets the S bit of the RJSS
array to 1. For chaptern[], the B bit is also set to 1. For the
variable-length chapters (chaptern[] and chapterc[]), the sender
checks the size variable to determine the bitfield length.

Before copying chaptern[], the sender updates the Y bit of each note
log to code the onset of the associated NoteOn command (Figure A.6.3
in [RFC4695]). To determine the Y bit value, the sender checks the
note_tstamp[] array for note timing information.

5.3. Updating the RJSS

After an RTP packet is sent, the sender updates the RJSS to refresh
the checkpoint history (Step 5 of the sending algorithm defined in
Figure 8). For each command in the MIDI list of the sent packet, the
sender performs the update procedure we now describe.

The update procedure begins at the leaf level. The sender generates
a new bitfield array for the chapter associated with the MIDI command
using the chapter-specific semantics defined in Appendix A of
[RFC4695].

For Chapter N and Chapter C, the sender uses the bitfield_ptr[] array
to locate and update an existing log for a note or controller. If a
log does not exist, the sender adds a log to the end of the
chaptern[] or chapterc[] bitfield and changes the bitfield_ptr[]
value to point to the log. For Chapter N, the sender also updates
note_tstamp[].

The sender also clears the S bit of the chapterp[], chapterw[], or
chapterc[] bitfield. For chaptern[], the sender clears the S bit or
the B bit of the bitfield, as described in Appendix A.6 of [RFC4695].

Next, the sender refreshes the upper levels of the RJSS hierarchy.
At the second level, the sender updates the cheader[] bitfield of the
channel associated with the command. The sender sets the S bit of
cheader[] to 0. If the new command forced the addition of a new
chapter or channel journal, the sender may also update other
cheader[] fields. At the top level, the sender updates the top-level
jheader[] bitfield in a similar manner.

Finally, the sender updates the seqnum variables associated with the
changed bitfield arrays. The sender sets the seqnum variables to the
extended sequence number of the packet.

5.4. Trimming the RJSS

At regular intervals, receivers send RTCP receiver reports to the
sender (as described in Section 6.4.2 of [RFC3550]). These reports
include the extended highest sequence number received (EHSNR) field.
This field codes the highest sequence number that the receiver has
observed from the sender, extended to disambiguate sequence number
rollover.

When the sender receives an RTCP receiver report, it runs the RJSS
trimming algorithm. The trimming algorithm uses the EHSNR to trim
away parts of the RJSS. In this way, the algorithm reduces the size
of recovery journals sent in subsequent RTP packets. The algorithm
conforms to the closed-loop sending policy defined in Appendix
C.2.2.2 of [RFC4695].

The trimming algorithm relies on the following observation: if the
EHSNR indicates that a packet with sequence number K has been
received, MIDI commands sent in packets with sequence numbers J <= K
may be removed from the RJSS without violating the closed-loop
policy.

To begin the trimming algorithm, the sender extracts the EHSNR field
from the receiver report and adjusts the EHSNR to reflect the
sequence number extension prefix of the sender. Then, the sender
compares the adjusted EHSNR value with seqnum fields at each level of
the RJSS, starting at the top level.

Levels whose seqnum is less than or equal to the adjusted EHSNR are
trimmed, by setting the seqnum to zero. If necessary, the jheader[]
and cheader[] arrays above the trimmed level are adjusted to match
the new journal layout. The checkpoint packet sequence number field
of jheader[] is updated to match the EHSNR.

At the leaf level, the sender trims the size of the variable-length
chaptern[] and chapterc[] bitfields. The sender loops through the
note_seqnum[] or control_seqnum[] array and removes chaptern[] or
chapterc[] logs whose seqnum value is less than or equal to the
adjusted EHSNR. The sender sets the associated bitfield_ptr[] to
null and updates the LENGTH field of the associated cheader[]
bitfield.

Note that the trimming algorithm does not add information to the
checkpoint history. As a consequence, the trimming algorithm does
not clear the S bit (and for chaptern[], the B bit) of any recovery
journal bitfield. As a second consequence, the trimming algorithm
does not set RJSS seqnum variables to the EHSNR value.

5.5. Implementation Notes

For pedagogical purposes, the recovery journal sender we describe has
been simplified in several ways. In practice, an implementation
would use enhanced versions of the traversing, updating, and trimming
algorithms presented in Sections 5.2-5.4.

6. Receiving Streams: General Considerations

In this section, we discuss receiver implementation issues.

To begin, we imagine that an ideal network carries the RTP stream.
Packets are never lost or reordered, and the end-to-end latency is
constant. In addition, we assume that all commands coded in the MIDI
list of a packet share the same timestamp (an assumption coded by the
"rtp_ptime" and "rtp_maxptime" values in Figures 1-2; see Appendix
C.4.1 of [RFC4695] for details).

Under these conditions, a simple algorithm may be used to render a
high-quality performance. Upon receipt of an RTP packet, the
receiver immediately executes the commands coded in the MIDI command
section of the payload. Commands are executed in the order of their
appearance in the MIDI list. The command timestamps are ignored.

Unfortunately, this simple algorithm breaks down once we relax our
assumptions about the network and the MIDI list:

If we permit lost and reordered packets to occur in the network,
the algorithm may produce unrecoverable rendering artifacts,
violating the mandate defined in Section 4 of [RFC4695].

If we permit the network to exhibit variable latency, the
algorithm modulates the network jitter onto the rendered MIDI
command stream.

If we permit a MIDI list to code commands with different
timestamps, the algorithm adds temporal jitter to the rendered
performance, as it ignores MIDI list timestamps.

In this section, we discuss interactive receiver design techniques
under these relaxed assumptions. Section 6.1 describes a receiver
design for high-performance Wide Area Networks (WANs), and Section
6.2 discusses design issues for other types of networks.

6.1. The NMP Receiver Design

The Network Musical Performance (NMP) system [NMP] is an interactive
performance application that uses an early version of the RTP MIDI
payload format. NMP is designed for use between universities within
the State of California, which use the high-performance CalREN2
network.

In the NMP system, network artifacts may affect how a musician hears
the performances of remote players. However, the network does not
affect how a musician hears his own performance.

Several aspects of CalREN2 network behavior (as measured in 2001
timeframe, as documented in [NMP]) guided the NMP system design:

The median symmetric latency (1/2 the round-trip time) of packets
sent between network sites is comparable to the acoustic latency
between two musicians located in the same room. For example, the
latency between Berkeley and Stanford is 2.1 ms, corresponding to
an acoustic distance of 2.4 feet (0.72 meters). These campuses
are 40 miles (64 km) apart. Preserving the benefits of the
underlying network latency at the application level was a key NMP
design goal.

For most times of day, the nominal temporal jitter is quite short.
For Berkeley-Stanford, the standard deviation of the round-trip
time was under 200 microseconds.

For most times of day, a few percent (0-4%) of the packets sent
arrive significantly late (> 40 ms), probably due to a queuing
transient somewhere in the network path. More rarely (< 0.1%), a
packet is lost during the transient.

At predictable times during the day (before lunchtime, at the end
of the workday, etc.), network performance deteriorates (10-20%
late packets) in a manner that makes the network unsuitable for
low-latency interactive use.

The NMP sender freely uses network bandwidth to improve the
performance experience. As soon as a musician generates a MIDI
command, an RTP packet coding the command is sent to the other
players. This sending algorithm reduces latency at the cost of
bandwidth. In addition, guard packets (described in Section 4.2) are
sent at frequent intervals to minimize the impact of packet loss.

The NMP receiver maintains a model of the stream and uses this model
as the basis of its resiliency system. Upon receipt of a packet, the
receiver predicts the RTP sequence number and the RTP timestamp (with
error bars) of the packet. Under normal network conditions, about
95% of received packets fit the predictions [NMP]. In this common
case, the receiver immediately executes the MIDI command coded in the
packet.

Note that the NMP receiver does not use a playout buffer; the design
is optimized for lowest latency at the expense of command jitter.
Thus, the NMP receiver design does not completely satisfy the
interoperability text in Appendix C.7.2 of [RFC4695], which requires
that receivers in network musical performance applications be capable
of using a playout buffer.

Occasionally, an incoming packet fits the sequence number prediction,
but falls outside the timestamp prediction error bars (see Appendix B
of [NMP] for timestamp model details). In most cases, the receiver
still executes the command coded in the packet. However, the
receiver discards NoteOn commands with non-zero velocity. By
discarding late commands that sound notes, the receiver prevents
"straggler notes" from disturbing a performance. By executing all
other late commands, the receiver quiets "soft stuck notes"
immediately and updates the state of the MIDI system.

More rarely, an incoming packet does not fit the sequence number
prediction. The receiver keeps track of the highest sequence number
received in the stream and predicts that an incoming packet will have
a sequence number one greater than this value. If the sequence
number of an incoming packet is greater than the prediction, a packet
loss has occurred. If the sequence number of the received packet is
less than the prediction, the packet has been received out of order.
All sequence number calculations are modulo 2^16 and use standard
methods (described in [RFC3550]) to avoid tracking errors during
rollover.

If a packet loss has occurred, the receiver examines the journal
section of the received packet and uses it to gracefully recover from
the loss episode. We describe this recovery procedure in Section 7
of this memo. The recovery process may result in the execution of
one or more MIDI commands. After executing the recovery commands,
the receiver processes the MIDI command encoded in the packet using
the timestamp model test described above.

If a packet is received out of order, the receiver ignores the
packet. The receiver takes this action because a packet received out
of order is always preceded by a packet that signalled a loss event.
This loss event triggered the recovery process, which may have
executed recovery commands. The MIDI command coded in the out-of-
order packet might, if executed, duplicate these recovery commands,
and this duplication might endanger the integrity of the stream.
Thus, ignoring the out-of-order packet is the safe approach.

6.2. High-Jitter Networks, Local Area Networks

The NMP receiver targets a network with a particular set of
characteristics: low nominal jitter, low packet loss, and occasional
outlier packets that arrive very late. In this section, we consider
how networks with different characteristics impact receiver design.

Networks with significant nominal jitter cannot use the buffer-free
receiver design described in Section 6.1. For example, the NMP
system performs poorly for musicians that use dial-up modem
connections, because the buffer-free receiver design modulates modem
jitter onto the performances. Receivers designed for high-jitter
networks should use a substantial playout buffer. References [GRAME]
and [CCRMA] describe how to use playout buffers in latency-critical
applications.

Receivers intended for use on Local Area Networks (LANs) face a
different set of issues. A dedicated LAN fabric built with modern
hardware is in many ways a predictable environment. The network
problems addressed by the NMP receiver design (packet loss and
outlier late packets) might only occur under extreme network overload
conditions.

Systems designed for this environment may choose to configure streams
without the recovery journal system (Appendix C.2.1 of [RFC4695]).
Receivers may also wish to forego or simplify the detection of
outlier late packets. Receivers should monitor the RTP sequence
numbers of incoming packets to detect network unreliability.

However, in some respects, LAN applications may be more demanding
than WAN applications. In LAN applications, musicians may be
receiving performance feedback from audio that is rendered from the
stream. The tolerance a musician has for latency and jitter in this
context may be quite low.

To reduce the perceived jitter, receivers may use a small playout
buffer (in the range of 100us to 2ms). The buffer adds a small
amount of latency to the system, which may be annoying to some
players. Receiver designs should include buffer tuning parameters to
let musicians adjust the tradeoff between latency and jitter.

7. Receiving Streams: The Recovery Journal

In this section, we describe the recovery algorithm used by the NMP
receiver [NMP]. In most ways, the recovery techniques we describe
are generally applicable to interactive receiver design. However, a
few aspects of the design are specialized for the NMP system:

The recovery algorithm covers a subset of the MIDI command set.
MIDI Systems (0xF), Poly Aftertouch (0xA), and Channel Aftertouch
(0xD) commands are not protected, and Control Change (0xB) command
protection is simplified. Note commands for a particular note
number are assumed to follow the typical NoteOn->NoteOff->NoteOn
->NoteOff pattern. The cm_unused and ch_never parameters in
Figures 1-2 specify this coverage.

The NMP system does not use a playout buffer. Therefore, the
recovery algorithm does not address interactions with a playout
buffer.

At a high level, the receiver algorithm works as follows. Upon
detection of a packet loss, the receiver examines the recovery
journal of the packet that ends the loss event. If necessary, the
receiver executes one or more MIDI commands to recover from the loss.

To prepare for recovery, a receiver maintains a data structure, the
Recovery Journal Receiver Structure (RJRS). The RJRS codes
information about the MIDI commands the receiver executes (both
incoming stream commands and self-generated recovery commands). At
the start of the stream, the RJRS is initialized to code that no
commands have been executed. Immediately after executing a MIDI
command, the receiver updates the RJRS with information about the
command.

We now describe the recovery algorithm in detail. We begin with two
definitions that classify loss events. These definitions assume that
the packet that ends the loss event has RTP sequence number I.

Single-packet loss. A single-packet loss occurs if the last
packet received before the loss event (excluding out-of-order
packets) has the sequence number I-2 (modulo 2^16).

Multi-packet loss. A multi-packet loss occurs if the last packet
received before the loss event (excluding out-of-order packets)
has a sequence number less than I-2 (modulo 2^16).

Upon detection of a packet loss, the recovery algorithm examines the
recovery journal header (Figure 8 of [RFC4695]) to check for special
cases:

If the header field A is 0, the recovery journal has no channel
journals, so no action is taken.

If a single-packet loss has occurred, and if the header S bit is
1, the lost packet has a MIDI command section with an empty MIDI
list. No action is taken.

If these checks fail, the algorithm parses the recovery journal body.
For each channel journal (Figure 9 in [RFC4695]) in the recovery
journal, the receiver compares the data in each chapter journal
(Appendix A of [RFC4695]) to the RJRS data for the chapter. If the
data are inconsistent, the algorithm infers that MIDI commands
related to the chapter journal have been lost. The recovery
algorithm executes MIDI commands to repair this loss and updates the
RJRS to reflect the repair.

For single-packet losses, the receiver skips channel and chapter
journals whose S bits are set to 1. For multi-packet losses, the
receiver parses each channel and chapter journal and checks for
inconsistency.

In the sections that follow, we describe the recovery steps that are
specific to each chapter journal. We cover 4 chapter journal types:
P (Program Change, 0xC), C (Control Change, 0xB), W (Pitch Wheel,
0xE), and N (Note, 0x8 and 0x9). Chapters are parsed in the order of
their appearance in the channel journal (P, then W, then N, then C).

The sections below reference the C implementation of the RJRS shown
in Figure 10. This structure is hierarchical, reflecting the
recovery journal architecture. At the leaf level, specialized data
structures (jrec_chapterw, jrec_chaptern, jrec_chapterc, and
jrec_chapterp) code state variables for a single chapter journal
type. A mid-level structure (jrec_channel) represents a single MIDI
channel, and a top-level structure (jrec_stream) represents the
entire MIDI stream.

7.1. Chapter W: MIDI Pitch Wheel (0xE)

Chapter W of the recovery journal protects against the loss of MIDI
Pitch Wheel (0xE) commands. A common use of the Pitch Wheel command
is to transmit the current position of a rotary "pitch wheel"
controller placed on the side of MIDI piano controllers. Players use
the pitch wheel to dynamically alter the pitch of all depressed keys.

The NMP receiver maintains the jrec_chapterw structure (Figure 10)
for each voice channel in jrec_stream to code pitch wheel state
information. In jrec_chapterw, val holds the 14-bit data value of
the most recent Pitch Wheel command that has arrived on a channel.
At the start of the stream, val is initialized to the default pitch
wheel value (0x2000).

At the end of a loss event, a receiver may find a Chapter W (Appendix
A.5 in [RFC4695]) bitfield in a channel journal. This chapter codes
the 14-bit data value of the most recent MIDI Pitch Wheel command in
the checkpoint history. If the Chapter W and jrec_chapterw pitch
wheel values do not match, one or more commands have been lost.

To recover from this loss, the NMP receiver immediately executes a
MIDI Pitch Wheel command on the channel, using the data value coded
in the recovery journal. The receiver then updates the jrec_chapterw
variables to reflect the executed command.

7.2. Chapter N: MIDI NoteOn (0x8) and NoteOff (0x9)

Chapter N of the recovery journal protects against the loss of MIDI
NoteOn (0x9) and NoteOff (0x8) commands. If a NoteOn command is
lost, a note is skipped. If a NoteOff command is lost, a note may
sound indefinitely. Recall that NoteOn commands with a velocity
value of 0 have the semantics of NoteOff commands.

The recovery algorithms in this section only work for MIDI sources
that produce NoteOn->NoteOff->NoteOn->NoteOff patterns for a note
number. Piano keyboard and drum pad controllers produce these
patterns. MIDI sources that use NoteOn->NoteOn->NoteOff->NoteOff
patterns for legato repeated notes, such as guitar and wind
controllers, require more sophisticated recovery strategies. Chapter
E (not used in this example) supports recovery algorithms for
atypical note command patterns (see Appendix A.7 of [RFC4695] for
details).

The NMP receiver maintains a jrec_chaptern structure (Figure 10) for
each voice channel in jrec_stream to code note-related state
information. State is kept for each of the 128 note numbers on a
channel, using three arrays of length 128 (vel[], seq[], and time[]).
The arrays are initialized to zero at the start of a stream.

The vel[n] array element holds information about the most recent note
command for note number n. If this command is a NoteOn command,
vel[n] holds the velocity data for the command. If this command is a
NoteOff command, vel[n] is set to 0.

The time[n] and extseq[n] array elements code information about the
most recently executed NoteOn command. The time[n] element holds the
execution time of the command, referenced to the local timebase of
the receiver. The extseq[n] element holds the RTP extended sequence
number of the packet associated with the command. For incoming
stream commands, extseq[n] codes the packet of the associated MIDI
list. For commands executed to perform loss recovery, extseq[n]
codes the packet of the associated recovery journal.

The Chapter N recovery journal bitfield (Figure A.6.1 in [RFC4695])
consists of two data structures: a bit array coding recently sent
NoteOff commands that are vulnerable to packet loss, and a note log
list coding recently sent NoteOn commands that are vulnerable to
packet loss.

At the end of a loss event, Chapter N recovery processing begins with
the NoteOff bit array. For each set bit in the array, the receiver
checks the corresponding vel[n] element in jrec_chaptern. If vel[n]
is non-zero, a NoteOff command or a NoteOff->NoteOn->NoteOff command
sequence has been lost. To recover from this loss, the receiver
immediately executes a NoteOff command for the note number on the
channel and sets vel[n] to 0.

The receiver then parses the note log list, using the S bit to skip
over "safe" logs in the single-packet loss case. For each at-risk
note log, the receiver checks the corresponding vel[n] element.

If vel[n] is zero, a NoteOn command or a NoteOn->NoteOff->NoteOn
command sequence has been lost. The receiver may execute the most
recent lost NoteOn (to play the note) or may take no action (to skip
the note), based on criteria we describe at the end of this section.
Whether the note is played or skipped, the receiver updates the
vel[n], time[n], and extseq[n] elements as if the NoteOn executed.

If vel[n] is non-zero, the receiver performs several checks to test
if a NoteOff->NoteOn sequence has been lost.

If vel[n] does not match the note log velocity, the note log must
code a different NoteOn command, and thus a NoteOff->NoteOn
sequence has been lost.

If extseq[n] is less than the (extended) checkpoint packet
sequence numbed coded in the recovery journal header (Figure 8 of
[RFC4695]), the vel[n] NoteOn command is not in the checkpoint
history, and thus a NoteOff->NoteOn sequence has been lost.

If the Y bit is set to 1, the NoteOn is musically "simultaneous"
with the RTP timestamp of the packet. If time[n] codes a time
value that is clearly not recent, a NoteOff->NoteOn sequence has
been lost.

If these tests indicate a lost NoteOff->NoteOn sequence, the receiver
immediately executes a NoteOff command. The receiver decides if the
most graceful action is to play or to skip the lost NoteOn, using the
criteria we describe at the end of this section. Whether or not the
receiver issues a NoteOn command, the vel[n], time[n], and extseq[n]
arrays are updated as if it did.

Note that the tests above do not catch all lost NoteOff->NoteOn
commands. If a fast NoteOn->NoteOff->NoteOn sequence occurs on a
note number with identical velocity values for both NoteOn commands,
a lost NoteOff->NoteOn does not result in the recovery algorithm
generating a NoteOff command. Instead, the first NoteOn continues to
sound, to be terminated by the future NoteOff command. In practice,
this (rare) outcome is not musically objectionable.

The number of tests in this resiliency algorithm may seem excessive.
However, in some common cases, a subset of the tests is not useful.
For example, MIDI streams that assigns the same velocity value to all
note events are often produced by inexpensive keyboards. The vel[n]
tests are not useful for these streams.

Finally, we discuss how the receiver decides whether to play or to
skip a lost NoteOn command. The note log Y bit is set if the NoteOn
is "simultaneous" with the RTP timestamp of the packet holding the
note log. If Y is 0, the receiver does not execute a NoteOn command.
If Y is 1, and if the packet has not arrived late, the receiver
immediately executes a NoteOn command for the note number, using the
velocity coded in the note log.

7.3. Chapter C: MIDI Control Change (0xB)

Chapter C (Appendix A.3 in [RFC4695]) protects against the loss of
MIDI Control Change commands. A Control Change command alters the
7-bit value of one of the 128 MIDI controllers.

Chapter C offers three tools for protecting a Control Change command:
the value tool (for graded controllers such as sliders), the toggle
tool (for on/off switches), and the count tool (for momentary-contact
switches). Senders choose a tool to encode recovery information for
a controller and encode the tool type along with the data in the
journal (Figures A.3.2 and A.3.3 in [RFC4695]).

A few uses of Control Change commands are not solely protected by
Chapter C. The protection of controllers 0 and 32 (Bank Select MSB
and Bank Select LSB) is shared between Chapter C and Chapter P
(Section 7.4).

Chapter M (Appendix A.4 of [RFC4695]) also protects the Control
Change command. However, the NMP system does not use this chapter,
because MPEG 4 Structured Audio [MPEGSA] does not use the controllers
protected by this chapter.

The Chapter C bitfield consists of a list of controller logs. Each
log codes the controller number, the tool type, and the state value
for the tool.

The NMP receiver maintains the jrec_chapterc structure (Figure 10)
for each voice channel in jrec_stream to code Control Change state
information. The value[] array holds the most recent data values for
each controller number. At the start of the stream, value[] is
initialized to the default controller data values specified in
[MPEGSA].

The count[] and toggle[] arrays hold the count tool and toggle tool
state values. At the start of a stream, these arrays are initialized
to zero. Whenever a Control Command executes, the receiver updates
the count[] and toggle[] state values, using the algorithms defined
in Appendix A.3 of [RFC4695].

At the end of a loss event, the receiver parses the Chapter C
controller log list, using the S bit to skip over "safe" logs in the
single-packet loss case. For each at-risk controller number n, the
receiver determines the tool type in use (value, toggle, or count)
and compares the data in the log to the associated jrec_chapterc
array element (value[n], toggle[n], or count[n]). If the data do not
match, one or more Control Change commands have been lost.

The method the receiver uses to recover from this loss depends on the
tool type and the controller number. For graded controllers
protected by the value tool, the receiver executes a Control Change
command using the new data value.

For the toggle and count tools, the recovery action is more complex.
For example, the Damper Pedal (Sustain) controller (number 64) is
typically used as a sustain pedal for piano-like sounds and is
typically coded using the toggle tool. If Damper Pedal (Sustain)

Control Change commands are lost, the receiver takes different
actions depending on the starting and ending state of the lost
sequence, to ensure that "ringing" piano notes are "damped" to
silence.

After recovering from the loss, the receiver updates the value[],
toggle[], and count[] arrays to reflect the Chapter C data and the
executed commands.

7.4. Chapter P: MIDI Program Change (0xC)

Chapter P of the recovery journal protects against the loss of MIDI
Program Change (0xC) commands.

The 7-bit data value of the Program Change command selects one of 128
possible timbres for the channel. To increase the number of possible
timbres, Control Change (0xB) commands may be issued prior to the
Program Change command to select a "program bank". The Bank Select
MSB (number 0) and Bank Select LSB (number 32) controllers specify
the 14-bit bank number that subsequent Program Change commands
reference.

The NMP receiver maintains the jrec_chapterp structure (Figure 10)
for each voice channel in jrec_stream to code Program Change state
information.

The prognum variable of jrec_chapterp holds the data value for the
most recent Program Change command that has arrived on the stream.
The bank_msb and bank_lsb variables of jrec_chapterp code the Bank
Select MSB and Bank Select LSB controller data values that were in
effect when that Program Change command arrived. The prognum_qual,
bank_msb_qual, and bank_lsb_qual variables are initialized to 0 and
are set to 1 to qualify the associated data values.

Chapter P fields code the data value for the most recent Program
Change command, and the MSB and LSB bank values in effect for that
command.

At the end of a loss event, the receiver checks Chapter P to see if
the recovery journal fields match the data stored in jrec_chapterp.
If these checks fail, one or more Program Change commands have been
lost.

To recover from this loss, the receiver takes the following steps.
If the B bit in Chapter P is set (Figure A.2.1 in [RFC4695]), Control
Change bank commands have preceded the Program Change command. The
receiver compares the bank data coded by Chapter P with the current
bank data for the channel (coded in jrec_channelc).

If the bank data do not agree, the receiver issues Control Change
commands to align the stream with Chapter P. The receiver then
updates jrec_channelp and jrec_channelc variables to reflect the
executed command(s). Finally, the receiver issues a Program Change
command that reflects the data in Chapter P and updates the prognum
and qual_prognum fields in jrec_channelp.

Note that this method relies on Chapter P recovery to precede Chapter
C recovery during channel journal processing. This ordering ensures
that lost Bank Select Control Change commands that occur after a lost
Program Change command in a stream are handled correctly.

8. Security Considerations

Security considerations for the RTP MIDI payload format are discussed
in the Security Considerations section of [RFC4695].

9. IANA Considerations

IANA considerations for the RTP MIDI payload format are discussed in
the IANA Considerations section of [RFC4695].

10. Acknowledgements

This memo was written in conjunction with [RFC4695], and the
Acknowledgements section of [RFC4695] also applies to this memo.

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